Pollution control devices are universally employed on motor vehicles to control atmospheric pollution. Two types of devices are currently in widespread use-catalytic converters and diesel particulate filters or traps. Catalytic converters contain a catalyst, which is typically coated onto a monolithic structure in the converter. The catalyst oxidizes carbon monoxide and hydrocarbons, and reduces of the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution. Diesel particulate filters or traps are wall flow filters which have honeycombed monolithic structures typically made from porous crystalline ceramic materials.
In the state of the art construction of these devices, each type of the these devices has a metal housing which holds within it a monolithic structure or element that can be metal or ceramic, and is most commonly ceramic. The ceramic monolith generally has very thin walls to provide a large amount of surface area so it is fragile and susceptible to breakage. It also has a coefficient of thermal expansion generally an order of magnitude less than the metal (usually stainless steel) housing in which it is contained. In order to avoid damage to the ceramic monolith from road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and the metal housing, ceramic mat or paste materials are typically disposed between the ceramic monolith and the metal housing.
The process of placing or inserting the mounting material is also referred to as canning and includes such processes as injecting a paste into a gap between the monolith and the metal housing, or wrapping a sheet or mat material around the monolith and inserting the wrapped monolith into the housing.
Typically, the mounting materials include inorganic binders, inorganic fibers that may also serve as a binder, intumescent materials, and optionally, organic binders, fillers, and other adjuvants. The materials are used as pastes, sheets, and mats. Ceramic mat materials, ceramic pastes, and intumescent sheet materials useful for mounting the monolith in the housing are described in, for example, U.S. Pat. Nos. 3,916,057 (Hatch et al.), 4,305,992 (Langer et al.), 4,385,135 (Langer et al.), 5,254,410 (Langer et al.), and 5,242,871 (Hashimoto et al.).
One of the shortcomings of the state of the art pastes and mats used for mounting is that the edges are subject to erosion from the pulsating hot exhaust gases. Under severe conditions, over a period of time, the mounting materials can erode and portions of the materials can be blown out. In time the mounting materials can fail to provide the needed protection the monolith.
Solutions to the problem include the use of a stainless steal wire screen (see e.g., U.S. Pat. No. 5,008,086 (Merry)) and braided or rope-like ceramic (i.e., glass, crystalline ceramic, or glass-ceramic) fiber braiding or metal wire material (see, e.g., U.S. Pat. No. 4,156,333 (Close et al.)), and edge protectants formed from compositions having glass particles (see, e.g., EP 0 639 700 A1 (Stroom et al.)) to protect the edge of the intumescent mat from erosion by exhaust gases.
While each of the various approaches to edge protection has its own utility, there remains an ongoing need to reduce the erosion of the mat materials used to mount the monoliths.